Power electronic devices can reach performances that can cause cooling problems due to dissipated heat. The power of such devices, for example switching elements or the like, has increased in recent years and consequently the emitted heat has also increased. Two-phase cooling circuits are efficient in cooling of such electronic devices. The liquid is heated by the emitted heat of the power devices to be cooled and in contact with the cooling circuit until the liquid reaches its boiling temperature. The temperature of the liquid during evaporation is constant and thereby limits the maximum temperature the device to be cooled can reach.
The cooling circuit itself can comprise an evaporator and a condenser connected thereto in a closed circuit. In the evaporator there is arranged the liquid reservoir which is in thermal contact with the heat emitting device. The cross section of the evaporator is large enough to constitute the liquid reservoir or pool. The heat of the device causes the liquid to boil and thus vapor is generated and conveyed from the evaporator to the condenser. Within the condenser the vapor is cooled down again to fall below the boiling temperature. As a consequence thereof another phase change occurs and the vapor condenses to become a liquid again. The liquid is afterwards fed back to the evaporator and is stored within the liquid reservoir again. A cooling circuit working in accordance with the above-mentioned principle is explained for example in U.S. Pat. No. 5,195,577, the disclosure of which is hereby incorporated by reference in its entirety.
In such cooling circuits, the evaporator can serve as a liquid reservoir as explained above. This is achieved by constructing an inner volume of the evaporator which serves as the liquid reservoir. This liquid reservoir can be brought into thermal contact with a heat emitting device. When heat is transferred to the liquid therefore pool boiling can occur. With such pool boiling, the heat transfer performance of such an evaporator can be relatively poor, the evaporator can be quite bulky, the cooling circuit involves a large fluid inventory, and the cooling circuit can be difficult to make leak proof at high pressure conditions.
Attempts to address known drawbacks have included reducing the diameter of the evaporator by, for example, constituting the evaporator with a number of tube-like channels with a small diameter. When the diameter of these channels is below a critical value which is defined by the type of liquid used, the operation changes from “pool boiling” to “convection boiling” or “flow boiling”. When flow boiling or convection boiling occurs, a mixture of a gas phase or a vapor phase and a liquid phase is given at the exit side of the evaporator. Such convection boiling can improve the performance of an evaporator. But on the other hand such convection boiling can have the disadvantage that the performance of the condenser is affected in a negative way if it is fed with a mixture of a gas phase and a liquid phase. Thus, in order not to lose overall performance of the cooling circuit, it can be desirable to provide a pure vapor phase to the condenser.